This invention relates to a control circuit for an electromagnetic device more particularly but not exclusively, an electromagnetically operable fuel control valve forming part of the fuel system of a vehicle internal combustion engine, the control circuit comprising first and second terminals connected to the positive and negative terminals of a source of DC supply, a first controllable switch connected in series between one end of a winding forming part of the device and the first terminal, a second controllable switch connected in series between the other end of the winding and said second terminal, a first diode connected between said one end of the winding and said second terminal, a second diode connected between said other end of the winding and the first terminal, and means for controlling the conduction of said switches whereby when it is required to actuate the device the current in the winding is allowed to rise to a high value and is then allowed to fall to a lower value after which it is maintained for a period at a mean level by chopping action, until it is turned off to de-actuate the device.
In a known arrangement the voltage of the source of DC supply is approximately 90 volts and this is derived using a DC/DC boost converter from the 12 volt supply of the vehicle driven by the engine. The use of the higher voltage supply has a number of advantages as compared with a 12 volt supply but a disadvantage is the need to provide the converter which includes a transformer, switches, rectifiers and a control circuit. In the operation of the known circuit both switches are closed to achieve a rapid rise in the current flow and then one of the switches is opened followed by the other, this achieving when the one switch is opened current recirculation in one of the diodes and therefore a slow rate of current decay and when both switches are open, a more rapid rate of current decay with energy being fed back to the supply. At the predetermined low value of current flow both switches are closed until the current increases to slightly above the mean hold value and then the one switch is opened to allow slow current decay until the current falls slightly below the mean hold value, the one switch then being turned on and off to provide the chopping action. Finally both switches are opened to allow a rapid fall of the current to zero when it is required to de-actuate the device.
With the known circuit energy recovery can take place only when both switches are opened and the current is decaying rapidly. During the period of current chopping, energy is drawn from the source of supply but is dissipated as heat in the winding resistance and in said one diode and the second switch.